1. Field of the Invention
This invention relates to the preparation of polysaccharides having improved flow properties for use in enhanced oil recovery techniques.
2. Description of the Prior Art
In describing the work of others herein we do not admit that such work is actually prior art under 35 USC 102 or 35 USC 103 or that the work was prior in time to the making of the invention described and claimed herein. We reserve the right to establish a date of conception or reduction to practice prior to the effective date of any publication, patent, or work herein described.
Within recent years polysaccharides have been proposed for use in enhanced oil recovery processes. In micellar flooding techniques, for example, a high viscosity aqueous polysaccharide solution containing surfactants, co-surfactants, or coagents is injected under pressure through an injection well into an oil-bearing formation. The injected fluid flows toward one or more production wells carrying with it or pushing before it a portion of previously unrecovered crude oil. Polysaccharides function as thickening agents in the injected solution to provide high viscosity to control the mobility of the injected fluid through the porous formation. Polysaccharides decrease "fingering" of the flow (the preferential higher velocity flow along wider, more permeable pathways) so that the injected fluid contacts a large portion of the petroleum in place, conducting it toward the production well. Fingering is a major problem, particularly in formations which are interspersed with microscopic cracks and in which pores holding oil are in the 0.1-10 micron range. In such formations it is important that the injected fluid flow through the small pores without plugging while still having sufficient viscosity to flow slowly through large cracks in the strata. Polysaccharides which have been proposed for use in enhanced oil recovery include xanthan gum, scleroglucan, and polyglucosylglucan.
A number of techniques for chemically treating polysaccharides have been reported. For the most part these treatments have been aimed at purification or clarification. For example, in U.S. Pat. No. 3,316,241 Leder et al. describe a process for recovering xanthan gum from its fermentation medium which involves the dispersion of the gum in an organic solvent. Cationic treatment to purify biopolymers from solution has been reported by several workers. See for example McNeeley et al., U.S. Pat. No. 3,232,929; O'Connell, U.S. Pat. No. 3,355,477; and Patton et al., U.S. Pat. No. 3,382,229. Treatment with alkali metal salts is described in Buchanan et al., U.S. Pat. No. 3,773,752. Quaternary amine precipitation for purifying polysaccharides is described in Rogovin et al, U.S. Pat. No. 3,119,812 and Gill et al., U.S. Pat. No. 3,422,085. Lindblom et al. in U.S. Pat. No. 3,163,602 describe a method of preparing substituted heteropolysaccharides involving the reaction of fermenter broth with quaternary ammonium compounds. In U.S. Pat. No. 3,729,460 Patton describes the alkaline treatment of a heteropolysaccharide for clarification. The clarification was thought to be due to deacetylation and also some degree of depolymerization.
Enzyme treatments to decrease the plugging tendencies of polysaccharides have recently been reported by Colegrove in U.S. Pat. No. 4,010,071 and Wellington in U.S. Pat. No. 4,119,491. Colegrove and Wellington each describes a protease enzyme treatment for enzymes of microbiological origin. The protease enzymes were said to specifically attack cell-wall fragments suspended in solution, thereby reducing the tendency of these fragments to plug porous formations.